Different methods for antenna tracking have been devised for a variety of applications and system requirements. In a process commonly referred to as xe2x80x9cprogram track,xe2x80x9d the antenna pointing relative to the signal direction is known with sufficient precision that the antenna is commanded in a open loop manner to point at the signal source and follow known changes in the signal source direction. At the other extreme, xe2x80x9cclosed loop monopulse trackingxe2x80x9d designs are implemented in high performance tracking radars to determine the angular location of a target with as high a precision as possible. In this case, the target trajectories are unknown, and indeed the purpose of the tracking radar is to determine their trajectories. Unfortunately, this angular precision in locating radar targets is accompanied by significant design and calibration complexity and high cost. In between these extremes lies another tracking method, referred to as xe2x80x9cstep track,xe2x80x9d that starts with an estimated signal source direction to derive an open loop command to point in the estimated signal direction and then verifies correct antenna pointing. The step track method verifies correct antenna pointing by commanding angular offset positions about the nominal pointing direction and sensing the signal power at each commanded position. The differences in these measured signal values are processed to correct the antenna pointing from its nominal estimated position to one that aligns the peak antenna pattern level with the signal to achieve the highest signal level from the system.
The step track technique is commonly used for antenna alignment with signals whose positions are relatively fixed such as earth terminals used with geosynchronous satellites. Communication satellites (unlike radar targets) provide a steady signal level over the time periods needed for antenna pointing alignment. In addition, the satellite trajectory is well known within the accuracy of the ephemeris of the satellite. The user determines an estimated antenna pointing direction from knowledge of his location and the orbital location of the satellite. The antenna is then sequentially moved plus and minus for a predetermined amount in the azimuth direction and the power received at each point is measured. If the power levels are identical for both angular offsets, the antenna is correctly aligned with the signal. If the power levels differ at the two angular offsets, then the power level difference can be used to correct the antenna pointing. Having correctly determined the pointing alignment in the azimuth plane, the process can be repeated in the elevation plane. Such a process is commonly used for simple antennas that are used, for example, for direct broadcast television (TV) reception. These antennas have relatively broad beamwidths in comparison with the variation in the position of the satellite so that the antenna once aligned can be fixed in place.
Other applications, however, exist where the signal source direction dynamically varies and the antenna must follow this dynamic variation to maintain signal reception. The variation in the signal direction can also be sufficiently dynamic that the assumption of a static signal direction during the required measurement time for the conventional step track method is no longer valid. Because the signal source direction changes during the measurement time, valid measurement of the power levels at the offset positions cannot be made and hence the antenna position cannot be aligned with the signal direction. An example of this situation occurs with meteorological earth resource satellites that are in a relatively low earth orbit to obtain high resolution of meteorological features and have sufficient orbital motion to obtain global coverage of weather events. This present invention addresses the need to provide an accurate, reliable method of aligning an antenna with a signal source and following its dynamic motion while the signal is in view of the antenna.
According to an embodiment of the present invention, a method of tracking a moving signal source is realized in a tracking technique that exploits estimated variations in the dynamic motion of the signal source. In an embodiment of the present invention, open loop commands are relied upon for antenna positioning and received signal levels in offset positions are measured to derive data for correcting antenna pointing in consideration of a predetermined estimate of the time variation of the signal direction. Open loop commands reposition the antenna to offset positions in such a way that the pointing takes into account the position of the signal source at different times in its angular variation. Unlike the conventional step track method, the signal sampling of the angular offsets varies with the rate at which the signal direction varies. For this reason, the present invention can be referred to as xe2x80x9crate correction step trackxe2x80x9d because the dynamics of the signal directional changes are incorporated into the antenna tracking method. In an embodiment of the present invention, signal power measurements are used not only to derive corrections to the antenna pointing but also in reaching decisions on when to revalidate the step track alignment. Unlike the conventional step track technique where the angular offsets are in fixed orthogonal directions, e.g. azimuth and elevation, in an embodiment of the present invention, the angular offsets are along and orthogonal to the direction of the signal source motion, i.e., in-track and cross-track.
In an embodiment of the present invention, first it is verified that the antenna position accurately follows the commanded values and follows the positions given in a test trajectory. The next step uses an ephemeris trajectory and aligns the antenna with the satellite signal as the satellite clears the horizon. The offsets after this alignment serve to verify the correctness of the ephemeris trajectory at this point of the trajectory. The ephemeris trajectory is further validated at selected intervals during the satellite pass and is used to correct the pointing at these points along the trajectory. The differences between the ephemeris trajectory and the actual trajectory are noted and can subsequently be used to update the ephemeris trajectory. The signal sampling used in this process is taken in the in-track and cross-track coordinates of the antenna trajectory. Subsequent to a particular satellite pass, the differences between the actual and ephemeris trajectories can be used to validate the correctness of the ephemeris values and to identify the need for further adjustments to ephemeris accuracy and/or updating to current values. Thus, an embodiment of the present invention provides means for maintaining the antenna tracking when errors exist in the ephemeris trajectory and for determining required refinements in ephemeris values, thereby facilitating implementation of an open loop tracking technique for signal sources having a dynamic motion variation.
In accordance with an embodiment of the present invention, a method of tracking a signal from a moving signal source includes: processing variations in a received signal from a moving signal source to determine signal peak alignment values; and processing the signal peak alignment values and an a priori estimate of motion of the moving signal source to control an antenna to track the received signal. In one embodiment, the a priori estimate of motion includes position and rate estimates from an estimated ephemeris trajectory. In one embodiment, an open loop control system is used to control the antenna. In one embodiment, a rate programmer is used to control the antenna. For example, the rate programmer controls the antenna with in-track acceleration commands and/or differences between a measured trajectory and an estimated ephemeris trajectory are used to control the rate programmer. In one embodiment, the method further includes performing a tracking verification at a sample rate determined in consideration of the a priori estimate of motion, such that trajectory portions with more dynamic signal variations are sampled more frequently. In one embodiment, the method further includes using a step track measurement process to correct errors in the a priori estimate of motion.
In accordance with another embodiment of the present invention, a method of tracking a signal from a moving signal source includes performing an initial acquisition of a signal from a moving signal source by: positioning an antenna at a nominal azimuth angle and at an initial elevation angle relative to a horizon where the moving signal source is expected to appear, the initial elevation angle being selected such that a null between a main beam and a first sidelobe of the antenna is aligned with the horizon; and repositioning the antenna by providing azimuth offset commands corresponding to angular increments about the nominal azimuth angle, until the signal has been acquired by the antenna""s receiver using the antenna. In one embodiment, the angular increments in the signal sampling are a portion of a beamwidth of the antenna dependent on the tracking sensitivity coefficient constrained by the signal margin. In one embodiment, the method further includes employing a guard antenna to verify a main beam acquisition.
In accordance with another embodiment of the present invention, a method of tracking a signal from a moving signal source includes employing an open loop control system to steer an antenna by commanding angular offsets along and orthogonal to a direction of motion of a moving signal source. In one embodiment, the open loop control system is configured to implement an open loop step track process.
In accordance with another embodiment of the present invention, a method of tracking a signal from a moving signal source includes commanding angular offsets to control an antenna to track a signal from a moving signal source taking into consideration directional changes of the signal and comparisons between actual and estimated trajectories of the moving signal source. In one embodiment, a sampling of the angular offsets varies with a rate of change of signal direction. In one embodiment, the angular offsets are commanded in in-track and/or cross-track coordinates. For example, cross-track angular offsets are determined by open loop commanding the antenna about a cross-track direction. For example, in-track angular offsets are determined by open loop commanding the antenna ahead of the moving signal source and making a signal power measurement, and then open loop commanding the antenna behind the moving signal source correcting the in-track angular offset for an amount the moving signal source would proceed in an in-track direction during the intervening time. As another example, in-track angular offsets are determined by open loop commanding the antenna in front of the moving signal source, and slowing a tracking speed to let the moving signal source traverse an in-track antenna pattern and realign the in-track coordinate based on power variations in the signal. In one embodiment, the method further includes sampling the signal to validate a correctness of the estimated trajectory. For example, the signal is sampled depending upon an angular rate associated with the estimated trajectory at a particular point along the estimated trajectory. As a second example, the signal is sampled more frequently during portions of the estimated trajectory where signal trajectory dynamics are high. As a third example, the signal is sampled more frequently if deviations between the actual and estimated trajectories are determined.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.